The present invention relates to a control system for applying and releasing a vacuum from a work-gripping vacuum cup employed as the work-gripping element of a workpiece transporting or locating device. Such devices are widely employed in mass-production environments, typically to load and unload sheet metal parts into and from a die or to carry a part, such as an automobile windshield, to the vehicle to which it is to be installed.
Typical examples of prior art vacuum cup control systems of the type with which the present invention is concerned are found in U.S. Pat. Nos. 3,349,927; 3,568,959; 3,613,904; 3,712,415; and 4,453,755.
The foregoing patents employ a venturi passage which is connected to a source of air under pressure. Flow of air through the venturi passage induces a subatmospheric pressure in the throat of the venturi, and a passage connecting the venturi throat to the interior of the vacuum cup will induce a vacuum within the cup when the cup is applied to a workpiece surface. In the earlier of the patents identified above, it was necesary to maintain the flow of air through the venturi passage in order to maintain the vacuum in the cup because the air withdrawn from the cup flowed into the venturi passage and to the discharge vent at the end of this passage. Upon cessation of the air flow, air at atmospheric pressure was free to flow in the reverse direction through the discharge vent, venturi passage and into the vacuum cup to dissipate the vacuum. Efforts to address this problem involved the addition of structure which substantially increased the profile of the control device. This was a definite disadvantage in those applications where the cup is employed to load and unload a part from a die because the enlarged profile requires a larger die opening to provide clearance for movement of the part-handling device into and out of the die.
A second problem encountered with the earlier devices was that of quickly releasing the vacuum from the cup to release the workpiece at the conclusion of the handling operation. The passage from the discharge vent to the vacuum cup is a relatively restricted passage, and the rate of flow through this passage would diminish substantially as the pressure differential between the negative pressure within the cup and atmospheric pressure approached equalization.
In U.S. Pat. No. 4,453,755, an arrangement for conducting air from the air pressure source to the interior of the vacuum cup to achieve rapid release is disclosed; however, this approach again substantially increases the profile of the control device by the addition of a rather complex valving arrangement to the exterior of the venturi passage housing.
The present invention is directed to a control device for a vacuum cup which is of extremely compact exterior dimensions and which will automatically maintain an induced vacuum in the cup upon cessation of air flow through the venturi passage and which will achieve a rapid release of the vacuum when desired.